Process for producing thin polytetrahaloethylene films



June 23, 1953 D. B. PECK 2,642,625

- PROCESS FOR PRODUCING THIN ourmmmosmm: FILMS Filed June 23. 1950 BEATED ROLL 1N VENT 0R DB.PECK

I Patented June 23, 1953 PROCESS FOR PRODUCING THIN POLY-TETRAHALOETHYLENE FILMS David B. Peck, Williamstown, Mass., assignor toSpraguc Electric Company,

North Adams,

Mass., a corporation of Massachusetts Application June 23, 1950, SerialNo. 169,797

3 Claims.

This invention relates to a process for producing electrical insulationand more particularly relates to extremely thin flexible films ofpolytetraiiuoroethylene resin.

The high temperature stability of polytetrahaloethylenes and inparticular polytetrafluoroethylene has led to their use whereverpossible for high temperature insulating purposes. Hov ever, thefabrication and processing of these resins to give thin flexible layers,sheets and coatings have been at best difflcult. Thin layers of theresins may be deposited on metal and other surfaces by deposition ofresin particles from a suspension thereof, followed by sintering of theparticles. While these layers may be very thin, it is difiicult toremove them from the base to provide unsupported flexible films withoptimum properties. Films on the order of 0.0005" (.5 of 2, mil.) inthickness are extremely fragile when produced by the aforesaid methodand also do not possess the optimum electrical properties of the coldworked resin.

The preparation of thin films from such resins by extrusion methods hasnot met with much success. Polytetrafluoroethylene, once cured, cannotbe extruded into thin films by conventional processes. The extrusion ofpolytrifiuoromonochloroethylene is somewhat simpler the hightemperatures and pressures in volved render it also a process notadapted to commercial use.

Films of polytetrafiuoroethylene can be prepared in thicknesses as lowas about 0.002" (2 mils.) by a pressure sintering of a large block ofresin followed by shaving of a thin film of the resin block by use of alathe and microtome type cutting knife. These films are tough andrelatively useful where thicknesses on theorder of 2 mils. or more maybe tolerated. In the fabrication of electrical capacitors, however,dielectric thicknesses totaling 1 mil. or less are most frequentlydesired. And to make up this total thickness more than one layer ofdielectric is almost without exception employed in order tosubstantially eliminate the possibility of dead short circuit caused byconducting particles or holes in the dielectric layers. While it istherefore possible to build electrical capacitors stable at hightemperatures and which possess excellent electrical properties, it isnot possible to do so in anything approaching a reasonable cubic volume.This is particularly true in the case of capacitors to be operated at avoltage of 1000 or less wherein total dielectric thickness of 1 mil. isample.

It is an object of the present invention to overcome the foregoing andrelated disadvantages. A further object is to produce extremely thinpolytetrahaloethylene resin films by a simple and novel process; A stillfurther object is to produce polytetrafiuoroethylene resin films of athickness of about 0.5 mil.

These objects are attained in accordance wit the present inventionwherein there is employed about 1 mil. in thickness to high calenderroll pressure with an ambient temperature sufiicient to maintain theresin temperature at the point of calendering above the transition pointof the resin, that is the temperature at which the resin undergoes aphase change and a sharp drop in tensile strength.

In a more restricted sense, the invention is concerned with a processfor producing polytetrahaloethylene films less than about 1 mil. inthickness which comprises stretching films of said resin greater thanabout 1 mil. in thickness and calendering said stretched film under highpressure atan ambient temperature sufficient to maintain the resintemperature at the calendering point above the transition point of theresin.

In one of its preferred and limited embodiments, the invention isconcerned with a process for producing polytetrafiuoroethylene filmssubstantially less than 1 mil. in thickness which comprises stretching afilm of said resin of about 2 mils. thickness in a single direction to alength of at least about twice the original length in said direction andpassing said stretched film through high pressure calender rolls whoseaxes are perpendicular to the direction of said stretching and which aremaintained at an ambient temperature suificient to maintain thetemperature or" the resin film at the point of calendering above thetransition-point of the resin.

I have made the startling discovery that extremely thin films ofpolytetrahaloethylene resins and in particular polytetrafiuorethylenecan be made on a continuous high speed process which involvesessentially a pre-stretching of a moderately thick film of said resinfollowed by high pressure calendering under a special set of temperatureconditions. The pre-stretching is normally conducted in a singledirection and is carried out at a temperature below the transition jointof the resin. Thus it is in a sense a cold working of the resin,although normally the working. temperature is somewhat above room for,it appears that the frictional heat developed 7 within the resin at thecalendering line must not v be dissipated so rapidly that the resintemperature drops below the transition point. Thus the. hot calenderrolls, instead of serving to heat the resin, appear to serve as amoderately high temperature ambient medium preventing excessive loss offrictional heat. The calendering is advisably carried out with rollswhose axes are perpendicular to the line of resin stretching.

In addition to being able to prepare thin dielectric films by aninexpensive and continuous process under the present invention, I findthat the films so produced possess physical properties such as tensilestrength and electrical properties such as dielectric strength which arehigher per unit measure than that of the original resin film processed.

The types of resins which may be processed in accordance with thepresent invention are numerous. Among these resins are the polytetrahalo resins such as polytetrafluoroethylene andpolymonochlorotrifluoroethylene resins which are extremely difficult tofabricate in extremely thin films by prior processes. The normalstarting material applied in accordance with my invention is amoderately thick resin film normally from about 1 mil. to about 4 mils.in thickness which can be produced by shaving of the resin from a blockthereof, extruding, casting or other means. It is contemplated thatother resins such as the vinyls, styrene types, rubber and the likemight be processed in accordance with the invention.

The invention willjbe further described with reference to the appendeddrawing in which it represents the starting film which unwinds fromtension spool 11. The latter is provided with a brake arrangement tomaintain the proper tension on film l for the linear stretching' step.Film passes. over stretching roll it which is normally warm and is drawninto calendering rollsv l3 and I4. The driving speed of rolls l3 and. i4exceeds the. unwinding speed of spool ll in accordance with the desiredamount of stretch applied to the resin prior to the calendering. Rollsl3 and [4 are highly polished and bear against each other at very highpressures, ordinarily between 1 and 50 tons per lineal inch beingapplied to calender rolls having diameters of from 1 to 20 inches. Thetemperature of these calender rolls is sufficient to maintain the resinfilm at the point of calendering above its transition point. In the caseof thin polytetrafiuoroethylene films the roll temperature is normallybetween about 100 C. and 250 C. depending upon the calender rollpressure and the rolling speed. With polytrifluoromonochloroethylene;temperatures ordinarily between about 100 C. and 160 C. are maintainedas the transition point is lower with this resin than with thetetrafluoro resin. The. transition, temperature of. polytetrafluoroethybene resin is 32020., to 3219C. at atmospheric pressure. AthiEhBIJJP-IGSSHI'BS transition Jappears to take place at lowertemperatures. It appears that the frictional heat developed in the resinat the calendering point raises the film temperature above thetransition point.

The thin, flexible dielectric film l6 which passes from calender rollsI3 and 14 may be run over roll l5 which is normally cool and to take uproll I! which maintains a slight positive pull on film It to preventwrinkling of the latter.

In a typical experiment polytetrafluoroethylene film was prepared in athickness of 2 mils.

by shaving the film from a solid molded rod of the resin. This film,which is slightly opaque and cloudy in appearance, was processed inaccordance with the invention by passing over stretching roll l2 with atension of about 2 pounds per inch width of polytetrafiuoroethyleneresin to increase the length of the film by a factor of about 2.3.Stretching roll 12 was maintained at 150 C. The film at this point wasvery cloudy and structurally weak. The film was then passed through thecalendering rolls which were maintained at 150 C. by steam heat andunder a compressive force of 25 tons on 8 diameter rolls (sample was 2"wi e). The calendering speed, that is the speed of the film from thecalender, was about 60 feet per minute. Resulting film was transparentand very tough, possessing a thickness of about 0.6 mil. The dielectricstrength of this thin film as determined by a mercury immersion testvaried from about 2000 to 2600. volts per mil, the average being about2300.

A capacitor was made up from the filmv by winding two thin aluminumfoils with a total of four layers of the film. The resulting capacitorcould be operated at 200 C. without failure. Another capacitor of thesame type was placed in a metal tube whose inner diameter was the sameas the outer diameter of the capacitor winding and the resultingassembly heated at 375 C. for 25 minutes. After cooling, the capacitorslid from the metal tube and r the resin was found to be fused on theedges of the winding. Despite this fact, the capacitor was stillelectrically operative and possessed extreme capacity and temperaturestability.

Capacity i 0 25 0.0400 mfd. 0.0402

At each of these temperatures the power dissipation factor at 1000cycles was less than 0.01%.

Similar films have been prepared from 1.5 mil.polytrifiuoromonochloroethylene with a calendering and stretching rolltemperature of about C. The resultant film was clear and a'bout 0.5 mil.in thickness.

The'opacity of the disclosed polymers is reduced by my process to such adegree that they become highly transparent and resemble clearcellophane. If desired, the resin films may be laminated with metal onone or both sides during or after the thickness reducing step. Theinvention may also be practiced on resin films which are loaded withceramic particles and the like to eifect an increase in the dielectricconstant and/or to modify certain final properties such as opacity,thermoplastic flow and the like. Also, uniform layers of ceramic powdermay be introduced on the calender rolls and calendered into the resinfilm during the calendering step.

Resin films substantially less than 0.5 mil.

have been produced by the present invention and have made possible thefabrication of exceedingly small electrical capacitors and other relateddevices. Because of the temperature stability of thepolytetrahaloethylene resins the thin films may be metallized on one orboth sides for the preparation of metallized film capacitors withexceedingly small physical volume.

It is to be understood that the invention is not limited to a singlestretching and calendering operation as the film resulting from a singletreatment may be re-prooessed to form an even thinner dielectric sheet.

What is claimed is:

l. A process for producing polytetrafluoroethylene films containingceramic particles substantially less than 1 mil. in thickness Whichcomprises stretching a film of said resin containing ceramic particlesof about 2 mils. thickness in a single direction to a length of at leastabout twice the original length in said direction and passing saidstretched film through high pressure calender rolls Whose axes areperpendicular to the direction of said stretching and which aremaintained at an ambient temperature sufficient to maintain the resinfilm at the point of calendering above the transition point of theresin.

2. A process for producing polytetrafluoroethylene films substantiallyless than 1 mil. in thickness which comprises stretching a film of saidresin of about 2 mils. thickness in a single direction to a length of atleast about twice the original length in said direction and passing saidstretched film through high pressure calender rolls Whose axes areperpendicular to the direction of said stretching and which aremaintained at an ambient temperature sufiicient to maintain the resinfilm at the point of calendering above the transition point of theresin.

3. A process for producing polytetrahaloethylene films less than 1 mil.thick which comprises subjecting a resin film of about 2 mil. thicknessto a compressive calendering roll pressure of between 1 and 50 tons perlineal inch as applied to the resin film through the medium of severalcalendering rolls having a diameter of from 1 to 20 inches and having anambient temperature sufficient to maintain the resin temperature at thepoint of calendering above the transition point.

DAVID B. PECK.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,183,550 Deutschmann Dec. 19, 1939 2,223,833 Sander Dec. 3,1940 2,400,099 Brubaker et a1. May 14, 1946 2,406,127 Alfthan Aug. 20,1946 OTHER REFERENCES Ind. and Eng. Chem, pages 871-877, Sept. 1946.

1. A PROCESS FOR PRODUCING POLYTETRAFLUOROETHYLENE FILMS CONTAININGCERAMIC PARTICLES SUBSTANTIALLY LESS THAN 1 MIL. IN THICKNESS WHICHCOMPRISES STRETCHING A FILM OF SAID RESIN CONTAINING CERAMIC PARTICLESOF ABOUT 2 MILS. THICKNESS IN A SINGLE DIRECTION TO A LENGTH OF AT LEASTABOUT TWICE THE ORIGINAL LENGTH IN SAID DIRECTION AND PASSING SAIDSTRETCHED FILM THROUGH HIGH PRESSURE CALENDER ROLLS WHOSE AXES AREPERPENDICULAR TO THE DIRECTION OF SAID STRETCHING AND WHICH AREMAINTAINED AT AN AMBIENT TEMPERATURE SUFFICIENT TO MAINTAIN THE RESINFILM AT THE POINT